g2301_2400.s2384_largest_palindromic_number.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g2301_2400.s2384_largest_palindromic_number;

// #Medium #String #Hash_Table #Greedy #2022_08_25_Time_26_ms_(100.00%)_Space_54.8_MB_(100.00%)

/**
 * 2384 - Largest Palindromic Number\.
 *
 * Medium
 *
 * You are given a string `num` consisting of digits only.
 *
 * Return _the **largest palindromic** integer (in the form of a string) that can be formed using digits taken from_ `num`. It should not contain **leading zeroes**.
 *
 * **Notes:**
 *
 * *   You do **not** need to use all the digits of `num`, but you must use **at least** one digit.
 * *   The digits can be reordered.
 *
 * **Example 1:**
 *
 * **Input:** num = "444947137"
 *
 * **Output:** "7449447"
 *
 * **Explanation:**
 *
 * Use the digits "4449477" from " **44494**  **7** 13 **7** " to form the palindromic integer "7449447".
 *
 * It can be shown that "7449447" is the largest palindromic integer that can be formed.
 *
 * **Example 2:**
 *
 * **Input:** num = "00009"
 *
 * **Output:** "9"
 *
 * **Explanation:**
 *
 * It can be shown that "9" is the largest palindromic integer that can be formed.
 *
 * Note that the integer returned should not contain leading zeroes.
 *
 * **Constraints:**
 *
 * *   1 <= num.length <= 10⁵
 * *   `num` consists of digits.
**/
public class Solution {
    public String largestPalindromic(String num) {
        int[] count = new int[10];
        int center = -1;
        StringBuilder first = new StringBuilder();
        StringBuilder second;
        for (char c : num.toCharArray()) {
            count[c - '0']++;
        }
        int c = 0;
        for (int i = 9; i >= 0; i--) {
            c = 0;
            if (count[i] % 2 == 1 && center == -1) {
                center = i;
            }
            if (first.length() == 0 && i == 0) {
                continue;
            }
            while (c < count[i] / 2) {
                first.append(String.valueOf(i));
                c++;
            }
        }
        second = new StringBuilder(first.toString());
        if (center != -1) {
            first.append(center);
        }
        first.append(second.reverse().toString());
        return first.length() == 0 ? "0" : first.toString();
    }
}